1. Field of the Invention
The present invention relates to a display device in which a thin film transistor is used at least in a pixel portion and a method for manufacturing the display device.
2. Description of the Related Art
In recent years, technology that is used to form a thin film transistor using a semiconductor thin film (with a thickness of several nm to several hundreds nm) that is formed over a substrate having an insulating surface has attracted attention. The thin film transistor is widely applied to electronic devices such as ICs and electro-optical devices. Development of a thin film transistor particularly as a switching element of an image display device has been accelerated.
As a switching element in an image display device, a thin film transistor using an amorphous semiconductor film, a thin film transistor using a polycrystalline semiconductor film, or the like is used. As a method for forming a polycrystalline semiconductor film, there is known a technique in which a pulsed excimer laser beam is shaped into a linear laser beam by an optical system and an amorphous silicon film is scanned and irradiated with the linear beam so that the amorphous silicon film is crystallized.
Further, as a switching element in an image display device, a thin film transistor using a microcrystalline semiconductor film is used (see Patent Document 1: Japanese Published Patent Application No. Hei4-242724 and Patent Document 2: Japanese Published Patent Application No. 2005-49832).
A known conventional method for manufacturing a thin film transistor is that an amorphous silicon film is formed over a gate insulating film; a metal film is formed thereover; and the metal film is irradiated with a diode laser beam to change the amorphous silicon film into a microcrystalline silicon film (see, for example, Non-Patent Document 1: Toshiaki Arai et al., SID 07 DIGEST, 2007, pp. 1370-1373). According to this method, the metal film formed over the amorphous silicon film is provided to convert optical energy of the diode laser beam into thermal energy and is needed to be removed in a later step to complete a thin film transistor. That is, the method is that an amorphous semiconductor film is heated only by conduction heating from a metal film to form a microcrystalline semiconductor film.